Capacitive sensor devices, or devices which measure capacitance or a change in capacitance, can be utilized in a wide variety of different fields. This is because various data or parameters that are sought by a given application can be derived based on capacitance or changes therein. For example, capacitive sensors can be used to detect touch, gesture or proximity input in human interface devices, to detect proximity of non-human physical objects, to detect presence and/or volume of water or other liquids, to detect motion, doors and windows for security applications, and any other application that exhibits some change in capacitance. While the following discussion will be directed to non-mechanical human interface devices for simplicity, it will be understood that the same discussions can be implemented for various other uses and applications.
Among various non-mechanical human interface devices used today, capacitive sensor devices are often used to detect and measure touch or proximity input. Typically, a capacitive touch sensor implements analog circuitry which measures changes in capacitance between two or more electrical wires caused by touch or proximity of a person's finger. The resulting analog signal representing the change in capacitance is digitized and post-processed to perform the preprogrammed task desired by the user. Although modern capacitive sensors may be adequate, new challenges surface when working with mobile or battery-operated devices.
Any battery-operated device shares the common goal of providing low power consumption. While capacitive sensors may be modified to reduce power consumption, the general trade-off or concern becomes an undesirable loss in the sensitivity of the capacitive sensor readouts. In a simple example, reductions in sampling frequencies may provide longer battery life, but such reductions may result in significantly delayed responses and frustrating experiences for the user. In addition to reducing power consumption, capacitive sensor circuits must also be cognizant of the sensitivity of the capacitive sensor. For instance, circuit noise, as well as variations in temperature, supply voltage, manufacturing tolerances, and the like, can also adversely affect the ability to detect capacitive sensor input.
There are various schemes available today which measure changes in capacitance sensor input. One technique employs a ring oscillator and a counter, which track changes in oscillator frequency and counter values that result when capacitance changes. A second technique measures a timing delay in a delay chain relative to a reference delay caused by changes in capacitance. A third technique measures changes in capacitance using a form of delta-sigma modulation. A fourth technique employs an operational amplifier, which is used to sample charge transfers between a capacitive sensor and a feedback capacitor. While each technique may provide some benefit, each also has room for improvement.
In particular, the first three conventional techniques noted above rely on overclocking, or require a system clock frequency that is well above the effective sampling rate of the capacitive sensor. While these techniques may provide responsive capacitive sensor circuitry, the overclocking involved is adverse to battery life and power consumption. The fourth conventional technique noted above involves charging the capacitive sensor twice per sample, and tasks the operational amplifier for analog signal processing of the sampling. This technique may provide some improvement over the first three techniques in terms of power consumption, and may help to prolong battery life. However, even the fourth technique can be improved upon to provide still longer battery life.
The present disclosure is directed at addressing one or more of the deficiencies and disadvantages set forth above. However, it should be appreciated that the solution of any particular problem is not a limitation on the scope of this disclosure or of the attached claims except to the extent expressly noted.